The overhead of this patch is small. A new path is added for the preorder keys, and OrderCheck node's additional cost is pretty low, given that it only compares two rows and stores only a single row (previous row seen), hence the memory footprint is minuscule.
We can eliminate the new node and put onus or having the right order on the user like we do with volatile setting of the function.
That is exactly what the new node does, since we are not re sorting right now in case the order is wrong. Please see my explanation upthread, OrderCheck node's primary purpose is to check for a user error in the result rows order. The onus right now to give correct order is on user.
